Tokyo Electric Power Co. said one of the reactor cores at its stricken Fukushima Dai-Ichi nuclear plant is more seriously damaged than previously thought, setting back the utility’s plan to resolve the crisis.

Fuel rods in the core of the No. 1 reactor are fully exposed, with the water level 1 meter (3.3 feet) below the base of the fuel assembly, Junichi Matsumoto, a general manager at the utility known as Tepco, told reporters at a briefing in Tokyo. Melted fuel has dropped to the bottom of the pressure vessel and is still being cooled, Matsumoto said.